Roger Federer advanced last night to the quarterfinals of the U.S. Open tennis tournament in New York. He's been on a bit of a tear this summer, even though in tennis player years he's old - 33. But he is part of a trend in men's tennis. NPR's social science correspondent Shankar Vedantam joined us to explain why.

SHANKAR VEDANTAM, BYLINE: I was talking with Stephanie Kovalchik. She's a statistician at the RAND Corporation. But more to the point, she's a big fan of Swiss star Roger Federer. In 2011, when Federer turned 30, Kovalchik noticed there was a lot of discussion about whether he would continue to compete. And she found this strange because in terms of performance, Federer was still clearly among the world's best players. So she decided take a closer look at the supposed age barrier when it came to professional tennis. She computed the ages of the top 100 players over time, and she discovered something very interesting.

STEPHANIE KOVALCHIK: In the '90s, the age of the top players was around 25 years old. But beginning in sort of the mid-early 2000s, we see an increase in the average age of the top players so that now the current cohort is about 28 years old. So there's been a three-year increase in the average age of players in the 21st century.

GREENE: So Shankar, if she's saying a lot of the top players are now, on average, 28 years old, that's got to mean there are players who are over 30 who are doing really well. Has 30 become the new 20?

VEDANTAM: (Laughter) We can only hope, David.

GREENE: Yeah.

VEDANTAM: I think, in practice, what this means is that the number of men's tennis players who are over the age of 30 who qualify for grand slam tournaments like the U.S. Open and Wimbledon has doubled or even tripled in recent years. So Federer, who's going to turn 33 this year, is actually not an outlier. He's actually part of a wave.

GREENE: And a wave that seems to be just in men's tennis, right?

This does not apply to women.

VEDANTAM: That's right. In fact, when Stephanie Kovalchik looks at the women's side, she doesn't see a similar increase in age among the top women's players. And she thinks this might be happening because in men's tennis, games are often decided with the best of five sets, whereas in women's tennis it's often the best of three. And Kovalchik thinks that's what's happening in tennis is that it's increasingly becoming a stamina event, and that might have a connection with the growing age of the top players. Here she is again.

KOVALCHIK: Sports that tend to require explosive movement, such as sprinters, generally have peak performance age around the early 20s, whereas sports that focus more on endurance generally see peak performance in the late 20s or maybe even early thirties.

GREENE: OK, couple things I want to ask you about, Shankar - first is haven't men's tennis matches always been five sets? Why is it becoming more a test of endurance nowadays?

VEDANTAM: Yeah, that's a good point. I asked Stephanie Kovalchik about this. She actually tossed out a couple of theories. One is that tennis players have focused so much more on physical endurance, that many of the players are getting to age 30 in much better physical tape than players a generation ago.

But it could also be that this has something to do with changes in racquet technology. What's happened is tennis rackets have changed so that you can now hit the ball very hard, but also with a lot of control. And what this means is that if you watch the U.S. Open right now, you'll see these long, baseline rallies going going back and forth.

GREENE: Long rallies that go on forever, yeah.

VEDANTAM: Right. And so Kovalchik thinks that as these rallies get longer and longer, the matches get longer and longer, and again, stamina gets to count for more than just strength.

GREENE: So it all makes sense that you and I got through this segment without breaking a sweat.

VEDANTAM: Speak for yourself, David.

(LAUGHTER)

GREENE: Shankar, thanks, as always.

VEDANTAM: Thanks, David.

GREENE: That's Shankar Vedantam. He regularly joins us to talk about social science research, and you can follow him on Twitter. He's @hiddenbrain. Transcript provided by NPR, Copyright NPR.